1. Field of the Invention:
The present invention relates to a method for producing a lead frame. More particularly, it is concerned with a method for producing a new lead frame which has sufficient mechanical strength despite its fine inner leads for connection to an IC.
2. Description of the Prior Art:
The recent trend toward the higher degree of integration necessitates ICs and LSIs having more pins than before. This, in turn, necessitates making the lead frame, especially inner leads, finer than before.
Conventional lead frames are usually produced from a metal sheet by die-punching. This method encounters difficulties in forming a pattern with a pitch smaller than 0.3 mm. Therefore, it is not applicable to a lead frame for an IC having more than 60 pins.
To cope with this situation, there has been developed a new method for producing a lead frame from a metal sheet by etching on both sides thereof. This method is illustrated in FIGS. 8(A) to 8(C). The first step of this method consists of forming a photoresist film b on both sides of a metal sheet a (0.15-0.25 .mu.m thick) of copper alloy or 42 alloy (Fe--Ni alloy), and exposing and developing the photoresist film b. The photoresist film b after development is shown in FIG. 8(A). It is to be noted that the photoresist film b on one side of the metal sheet is identical with the one on the other side of the metal sheet, and they are positioned such that they overlap with each other.
In the next step, the metal sheet a is etched on both sides thereof with ferric chloride, using the photoresist film b as a mask. The metal sheet after etching is shown in section in FIG. 8(B).
Finally, the photoresist film b is removed as shown in FIG. 8(C), and a desired lead frame is obtained.
A disadvantage of this method is that it is difficult to make the lead pitch smaller than the thickness of the metal sheet from which the lead frame is made, because side etching is inevitable. (Etching takes place not only in the thickness direction of the metal sheet but also in the direction perpendicular to the thickness direction.) The amount of side etching is about two-thirds of the normal etching depth as shown in FIGS. 8(B) and 8(C). Since side etching takes place at both sides, the total amount of side etching (etching depth.times.2/3.times.2) affects the lead pitch. The greater the amount of side etching, the greater the minimum value of lead pitch. To reduce the lead pitch, it is necessary to reduce the amount of side etching, and to reduce the amount of side etching, it is necessary to reduce the etching depth. The etching depth depends on the thickness of the metal sheet from which the lead frame is made. To be more specific, the etching depth is half the thickness of the metal sheet for lead frame. There is a relationship between the thickness of the lead frame metal sheet and the minimum lead pitch that can be attained, as shown in FIG. 9. To attain a small lead pitch, it is necessary to use a thin metal sheet for lead frame.
The minimum thickness of the lead frame metal sheet is limited by the mechanical strength required for the lead frame. Therefore, it is practically very difficult or impossible to make the lead pitch smaller than 0.22 mm. Nevertheless, a lead pitch smaller than 0.22 mm is required for the recent ICs having a higher degree of integration.
To meet this requirement, a lead frame is produced in two steps. In the first step, that part of the lead frame metal sheet which should have a small lead pitch (or the region where the tip of the inner lead is formed) is thinned by selective etching. In the second step, etching is carried out for the patterning of the lead frame. This method needs two steps of complex etching and hence leads to a higher production cost.
Etching by the conventional method varies in etching depth depending on the subtle change in etching conditions. In an extreme case, the region where the tip of the inner lead is formed becomes too thin or disappears due to excessive etching. Because of the varying etching thickness and unstable quality, the conventional method is not of practical use.
An attempt has been made to replace the lead frame by a polyimide tape with a pattern of thin copper foil formed thereon. Unfortunately, polyimide tape is very expensive and needs a special technique for its fabrication. Moreover, polyimide does not have sufficient mechanical strength.